1. Field of the Invention
This invention relates to thinned array radar system, and particularly to a system to dynamically focus such an array so as to provide the best possible resolution at every range of data received in sequence during the time interval of a radar return.
2. Description of the Prior Art
Bistatic thinned array radar systems such as described in U.S. Pat. Nos. 3,825,928 and 3,842,417 provide a highly simplified receive antenna because the antenna radiating elements are separated by a spacing of one wavelength or greater so as to require a minimum of elements. The receive antenna thus has a resultant multilobe receive pattern with a selected lobe being illuminated by the single main lobe of a separate transmit antenna. Because of the small number of receiving elements, a separate radar receiving module including a mixer may be provided for each antenna receiving element at which point the received signals are heterodyned to IF by a phase shifted local oscillator signal. The IF outputs of all elements or each subarray of elements in the receive antenna are coherently summed and combined. The combined signal is post IF amplified and detected to form the total receiver output signal. By corporately combining the received energy at IF frequencies rather than at RF (Radio Frequencies) the system has minimum signal losses and can employ coaxial cable rather than waveguides between the antenna elements and the receiver. Coaxial cables have the advantage that they may be mounted where space is limited such as in the wing of an aircraft. A similar approach can be taken for large, two dimensional, thinned arrays if the need arises for such a device. For point targets at very long ranges, the incoming wave to the receive antenna is approximately a plane surface but at shorter ranges this surface becomes spherical with a radius equal to the range R. When a significant phase error (.pi./2) is introduced across an antenna that is focused on a plane wave by the curvature of this spherical surface, the antenna is said to be operating in its near field and must be focused at every range to maintain its theoretical resolution of R.lambda./l where .lambda. is the wavelength and l is the antenna length. In most thinned array configurations the near field range is quite large being equal to l.sup.2 /2.lambda. which for example is 12,500 feet for the 50 foot long X-band antenna. To provide dynamic focusing at these shorter ranges during the processing, any phase correction must operate with great speed. For example, the phase at the end of the array must change at about 360.degree. per microsecond at a range of 1,000 feet for a 50 foot X-band antenna. Such rates of change are quite difficult to perform with phase shifters and unless done very quickly would tend to break up the radar imagery at the points of phase change. A simplified focusing system that does not require the complexity of high speed phase shifters and that would not distort the radar imagery would be a substantial advantage to the art.